1. Field of the Invention
The present invention relates to a paper size determination device for determining the size of a sheet of paper which is actually loaded in a printing operation, for example, and more particularly to a paper size determination device for determining the size of a sheet of paper by measuring the length of time required for the paper to pass a preset position on a paper feeding path.
2. Description of the Related Art
A laser printer or a copying machine are well known examples of an electrophotographic printing device. A typical electrophotographic printing device includes a photosensitive drum serving as an image carrier, and further includes processing sections disposed around the photosensitive drum, for effecting the charging, exposing, developing, charge transferring, cleaning, and discharging operations. The photosensitive drum is rotated during the printing operation and sequentially subjected to the processes performed by the above processing sections. The charging section uniformly charges the surface of the photosensitive drum, the exposing section selectively exposes the surface to create an electrostatic latent image corresponding to image data, the developing section supplies toner to be affixed to that portion of the surface which corresponds to the electrostatic latent image, so as to convert the electrostatic latent image to a visual image, and the charge transferring section charges a sheet of paper from a paper supplying cassette and set in the charge transferring position so as to transfer the toner image on the drum to the paper. Thereafter, the paper is discharged to the exterior via a fixing section for fixing the toner image on the paper. Then, the cleaning section removes toner remaining on the drum and the discharging section removes any charges remaining thereon.
The above electrophotographic printing device is constructed such that it determines the size of a sheet of paper actually loaded on the charge transferring section during a normal printing operation. Specifically, a paper sensor is provided for detecting a sheet of paper passing a preset position on the paper feeding path, and the paper detection period is measured by a microprocessor which is used to control the entire printing operation. In order to measure the paper detection period, the microprocessor repeatedly checks the paper sensor, operates an internal timer after the paper is detected until the absence of the paper is detected by the paper sensor, and determines the paper size based on the length of the paper, which is obtained by multiplying time data derived from the timer by the paper feeding speed. For example, if a printing operation is started without it being known that the loaded paper supplying cassette does not contain sheets of paper of correct size, unwanted paper is supplied to the paper feeding path from the paper supplying cassette, at which time it is determined that the length of the paper does not correspond to the correct paper size. Consequently, it is necessary to replace the paper supplying cassette before the next printing operation is started. The microprocessor sequentially controls the processing sections to create a toner image on the photosensitive drum, for example, in addition to checking the paper sensor. Therefore, the microprocessor must rapidly process various data necessary for each control operation so as to operate the processing sections at the proper timings. However, if the microprocessor has to check the paper sensor frequently in order to determine that the paper length is correct, this may delay the data processing operation. In order to prevent this problem from occurring, time allocation for the respective tasks must be set precisely, thus making it more difficult to create the required microprocessor program.
The conventional device uses a microprocessor having a relatively high processing ability so as to easily cope with future modifications made by software developing engineers. However, the cost of such a microprocessor including the peripheral circuits is quite high, making it difficult to manufacture the conventional device at a relatively low cost.